To simplify and increase the efficiency of fuel cell stacks of this type, unhumidified or partially humidified fuel cells are often used at high operating temperatures and low pressures. This eliminates the outlay on equipment for gas/gas humidification devices or makes it possible to dispense with the radiator surface area required for the condensation. On account of the high operating and therefore cooling water temperatures of the fuel cell, the vehicle radiator can be reduced in size, and the compressor power can be reduced on account of the low operating pressure.
Under the operating conditions described, the problem of the electrolyte of the membrane electrode assembly (MEA) being (partially) dried out—particularly at the cathode entry—by the unsaturated gas entering it inevitably arises.
The prior art is represented by special arrangements of a plurality of fuel cell stacks through which the reactants (generally gases) flow in succession. The water formed by the cell reaction in the first stack is thus entrained by the reactants to the subsequent stacks. An arrangement of this nature is described, for example, in EP1009050, in which the cathode gas of a first (low-temperature) stack is supplied with a gas mixture made up of cathode gas from a second (high-temperature) stack and fresh air which is metered in. However, this only allows the moisture content to be adapted at the entry to the second stack. Since there are no additional gas mixing features in this arrangement, neither the moisture level nor the oxygen or hydrogen partial pressure can be set locally, i.e. for example for individual cells within a stack. The result of this is that the oxygen or hydrogen partial pressure is significantly (often 1.5 to 3 times) greater at the passage entry than at the exit. This leads to an inhomogeneous reaction distribution and the risk of local overheating (known as hot spots). Moreover, at either the cathode or anode entry there is a risk of drying out, or at the exit there is a risk of condensation and therefore of the supply of starting materials to the reactive zones of the MEA being impeded.
Another proposal for influencing the moisture distribution forms the subject matter of DE 100 55 253 A1. In this arrangement (illustrated in FIG. 1), the distributor plate (A) of a fuel cell has a channel region (B) with a plurality of parallel gas passages (C). These gas passages (C) run from a port region (D) which is used to supply gas to a port region (E) via which the gas is discharged. Connecting passages (F) run between the port region (D) and the gas passages (C). It is possible to locally meter fresh, unused gas from the port region (D) into the gas passages (C) via these connecting passages (F). On account of the cross section of the connecting passages (F) being reduced compared to the gas passages (C), the volumetric flow of the fresh gas stream is metered in such a manner that the local humidity which is present in the gas passage (C) is sufficient to prevent the MEA from drying out. Since the connecting passages (F) are connected to a common feed line port (D), influencing of the gas composition always affects the entire cell.